Radiofrequency (RF) ablation is now being used for minimally invasive focal destruction of malignant tumors. Microwave ablation has many advantages over RF ablation, but has not been extensively applied clinically due to the large probe size (14 gauge) and relatively small zone of necrosis (1.6 cm in diameter) [Seki T, Wakabayashi M, Nakagawa T, et al. Ultrasonically guided percutaneous microwave coagulation therapy for small hepatocellular carcinoma. Cancer 1994;74:817-825] that is created by the only commercial device (Microtaze, Nippon Shoji, Osaka, Japan. 2.450 MHz, 1.6 mm diameter probe, 70 W for 60 seconds). The large probe size would not be compatible with percutaneous use in the chest, and would only be used with caution in the abdomen.
The basis of this invention is the resonant monopole antenna resulting from exposing a length of the center conductor equal to an electrical length equal to (2n−1)λ/4, where n=1,2 . . . 10, and is typically n=1. In this case, the electric field peaks at the end of the exposed center conductor, and is ideally at a minimum where the outer conductor terminates (FIG. 1).
The volumetric shape of the electric field lines will roughly define the zone of necrosis when a resonant coaxial probe is used as an ablation tool. More precisely, the cross product of electric and magnetic field lines determines the power available to deposit into the tissue surrounding the probe tip.
As an example, given the high dielectric constant of liver tissue (εr=43.03), the quarter-wave length of the center conductor protrusion for resonance at 2.45 GHz would be 4.66 mm, severely limiting the zone of necrosis. A three-quarter wave long protrusion (approximately 14 mm in physical length), which can be extended by multiple half-wavelengths is preferable.
The protrusion of the outer conductor of a coaxial line is also set to (2n−1)λ/4, where n=1,2 . . . , and is not necessarily the same amount as the inner conductor (FIG. 2). This enforces the zero-electric field boundary condition on the coaxial outer conductor, which becomes the “middle conductor.”